The invention relates to a method for the fine control of a hydrostatic/mechanical multiple path power transmission, which comprises an adjustable hydrostat unit, a summing planetary gearbox with two input shafts--one from the hydrostatic and one from the mechanical path--and at least two output shafts, and a step gearbox connected to the output shafts.
When agricultural tractors or work vehicles are used, it is desired for specific operations--for example for sowing or sweeping--to be able to travel for a relatively long time at a very low and precisely set speed or, in the case of difficult maneuvres--for example when coupling a work machine--to perform very slow movements, with occasional standstill, even on sloping terrain. In the case of the multiple path power transmissions known hitherto, these modes of operation are not possible.
A great problem here resides primarily in the fact that precise speed control needs a precise actual signal. However, such a signal is not available at low speeds. At a standstill, there is no signal available at all, for which reason standstill control is impossible. The signal from an inductive transmitter is too weak and the pulse train is too long, the amplitude of a signal from a Hall transmitter is certainly adequate but the pulse train is also again too slow. This causes agonizingly slow response to the actuation of the throttle lever (desired speed) and to changes in the driving values; in the extreme case, a molehill on the field can lead to stalling of the engine. Precise setting of the crawl speed is not possible.
Transmissions on which the method is based are disclosed, for example, by DE 195 27 754 A1 and U.S. Pat. No. 5,159,855. In the case of these transmissions, the summing planetary gearbox has five shafts. One input shaft is mechanically directly driven, the second via the hydrostat. Of the three output shafts, in each gear one conveys the drive torque into the subsequent step gearbox. Depending on which gear is selected there, the power flow is also different in the summing planetary gearbox, and so also is the transmission ratio in relation to the power flow. U.S. Pat. No. 5,159,885 even discloses that if a coupling shaft is intended to be set to zero rotational speed, this is not guaranteed, for measurement and control reasons, and that the motor vehicle may move unintentionally.
The object of the invention is therefore to provide a method according to which very low speeds, right down to a complete standstill, can be adjusted precisely and rapidly.